Engine compression

[speedyblue]

I just checked the compression in my engine with a compression gauge, holding the carb open and the throttle flat to the floor with a warm engine. Came up at 100 in all cylinders. Is this good for a stock L28 with a 8.3:1 static compresion ratio? I have a feeling it should be higher.

Any ideas?

[Fentin_Fury]

Just completed a c test last night - although different engine so comparison will be limited. On a '73 L24 with E88 head, cold, I found an even 170-180 across. Now the listed compression for this engine is ~8.75:1 - very similiar. 100 sounds low to me.

[speedyblue]

And that was cold did you say?

Warm the engine up and it will be higher again.

Well I guess that means I will have to do something about that. I will have to check the other engine I have and see what it comes up to. Hopefully better than 100.

[texasz]

As I understand it you should be getting about 170 in all cyls. This is what I recall being told here a few months back. 100 is low.

[Ed]

Originally posted by speedyblue

holding the carb open and the throttle flat to the floor with a warm engine.

Question, When I do a compression check I don't have the engine running. I pull all the plugs and just crank the engine over a couple of times until the needle on the guage doesn't move anymore.

Am I doing it wrong?

[Inf]

I'm pretty new at working on cars, so this is probably a dumb question:

Is it bad to remove the spark plugs (or other bolts for that matter) from the cylinder head when the engine has been warmed up? It seems to me that there would be some problems with the threads being damaged what with the different heat expansions of each metal...

Is it OK to do if i'm sure the threads have had some high temp anti-sieze put on them before?

[Fentin_Fury]

For a compression test (if it tells you anything - other than everything is the same - not where the problem might be)the throttle(s) should be wide open - then crank till the guage does not move anymore - hot or cold - I'm not sure. Hot should give you a higher number and a better indication.

Leak down tests are far more diagnostic - and should be done hot.

Speedyblue - the compression test I did was on a cold engine - and that engine burns oil - she's a smoker!

Q

[Tourniqet]

so you take all the plugs out and then crank it one cylinder at a time?

do you warm engine up and then take all the plugs out for better reading?

I just bought an old compression tester at an estate sale and would like to test it out.

[speedyblue]

Just because it is a smoker, doesn't mean the compression should be shot. The two top rings on the piston are the ones that keep the compression above the piston, while the two thin lower ones with the funny spring spacer thingy keep the oil out.

Sometimes they can wear strangely, so the oil rings are shot, but the compression rings are still good. Either that or your sump is pressurised somehow, helping the oil get past.

I have never really thought about removing/replacing hot bolts or plugs. But the expansion that goes on is very small, and both would expand a little, but at different rates. I guess seeing as the plugs still go in easily, there can't be too much damage going on.

That is how I tested mine. Warmed up the engin for 10-15 minutes, took out all the plugs, plus disconnected the power to the coil. Hold the throttle wide open, plus prop the su carb piston open so it doesn't hinder air flow. You don't have to take all the plugs out, you can do it one at a time, just make sure you disconnect the coil so it doesn't start up.

[Rusty1]

I have a book that explains the method of doing a compression test.

Yes, motor must be warm.

It recommends loosening all of the plugs but not removing them and turning the engine over to blow debris out of the plug holes prior to removal.

Take all of the plugs out, and complete the test as you previously mentioned.

There are a few things you can deduce from a compression test,

If a cylinder reads low, place a teaspoon of oil into the bore and turn the engine through ten compression strokes to spread the oil. Re-test, if the compression comes up then the rings are passing (the oil improves the seal).

If two cylinders side by side record similar low readings, you may have head gasket problems.

Higher than manufacturers specification readings are an indicator of carbon buildup on top of the piston and heads.

The books says that anything greater than a 10% variation indicates problems.

[speedyblue]

Well I checked the 280zx out and it ranged from 140 to 150 so it looks like the brakes aren't the only thing being scavenged from it.

One engine swap to go.